The invention relates to a method of manufacturing molded articles.
Mixtures of polypropylene and wood particles are known in specialist circles by the term HMPP (wood dust polypropylene). In this instance the workpiece mixture is produced in panel form by extrusion, if necessary with additional calendering. The wall thickness of these panels correspond to the thickness of the later molded article. The HMPP panels are heated by radiant heating to the softening temperature of the polypropylene. The panels are thereafter shaped by appropriate shaping procedures such as vacuum deep drawing, compression in a tool comprising male and female mold sections or the like, to produce the respective molded article required. In this case the temperature of the tools lies below the softening temperature of the polypropylene, so that during shaping, the material is cooled, and the molded article may be removed from the tool in a dimensionally stable condition.
This procedure in the shaping of HMPP panels has the advantage of simplicity and ease of monitoring the production process. However, a series of drawbacks stand in contrast to this advantage. For example, a high proportion of cost-intensive polypropylene is required in order to make the HMPP panels sufficiently flowable and thus deformable. According to the state of the art, the minimum proportion of polypropylene comes to more than 50%. This is not only disadvantageous for reasons of cost, but also has a negative effect on the properties of the molded articles. The dimensional strength of the molded articles in the higher-temperature range increases with an increasing proportion of wood particles, and its brittleness decreases in the low-temperature range. An increase in the proportion of wood particles would therefore be desirable not only for reasons of cost but for the properties of the molded articles. However through the fact that a larger proportion of wood particles reduces the deformability of the panels, articles of complex shape can no longer be produced.
A further disadvantage in the known HMPP panels resides in the fact that, due to their dense structure and their low heat-conductivity, they can only with difficulty be heated in a continuous manner to the softening temperature of the propylene. A relatively lengthy heating-time is required for this purpose.
Molded articles manufactured from HMPP panels are being increasingly used as internal trimming components for motor vehicles. HMPP panels have low water absorption because of the high proportion of binder. As a result, they are, in this field of the application, and despite their heavy reliance on their mechanical parameters, for example temperature resistance, an alternative to corresponding molded articles. Molded articles are shaped with a relatively low binder proportion of less than 20% by deformation of a matted fleece and subsequent pressing in a hot press tool. Such molded articles, however, have the great advantage that with regard to their strength values, they are largely independent of temperature. On the other hand, due to their low binder content, they have worse parameters relating to water absorption. In 1980 there was proposed a method of manufacturing compressible panel material from cellulose-containing material and at least one thermoplastic binder, the cellulose-containing material and the thermoplastic binder being admixed, and the mixture being shaped into panel material in a heated state. (DE-OS 28 45 117). Here the cellulose-containing material is pulped into fibrous material and mixed with the thermoplastic binder. A fleece is formed from the mixture, and the fleece is compressed to form panel material by the application of heat and pressure. This method was intended to enable the simple and cost-effective production of panel material characterized by good homogeneity and mechanical properties, which may be finally pressed in a simple way to produce high-quality molded articles. As cellulose-containing material, there was preferably used waste from cellulose fiber materials, which was communicated by chipping and pulped by dry milling, and adding thermoplastic binder in the form of dry powder.
It is also known from the prior art to add to the mixture duroplastic binders in the form of dry powder, the proportion by weight of the duroplastic binder preferably being 3 to 6% with respect to the overall proportion of binder.
Polyethylene, polypropylene, polyamide or the like may be used as a thermoplastic binder, conventionally with a weight proportion of more than 40%, and preferably 50 to 60%, so that the weight proportion of the fiber material is at most equal to the weight proportion of the binder.
This procedure does make it possible to transfer a series of advantages of fiber fleece mat technology to the HMPP technology, which is advantageous in manufacturing terms. However, it still has serious drawbacks. Even 14 years after this procedure became known, no relevant marketable product has been manufactured to date. The essential reasons for this can be summarized as follows:
In this method, a proportion of 50% by weight and over of thermoplastic material is required. At densities of 0.8 g/cm.sup.2 to 1.4 g/cm.sup.2, the molded articles have no further air permeability for corresponding molded articles, and therefore present problems during subsequent lamination processes.
The aim of incorporating duromeric binders--preferably in powder form--is an improvement in the dimensional stability of the molded articles under heat stress. In reality the heating-times before shaping are so long that the duromeric binder polymerizes out during the heating stage, and thus remains ineffective for the bond. During heating to softening temperature, the semifinished panels expand. The duromer reacts out in this condition, and is lost for reinforcement of the bond.
Of the large proportion of thermoplastic binders, only polypropylene is feasible in practice for technical and economic reasons, particularly with regard to the softening temperature. The large proportion of thermoplastic binders provides an added complication because of the separating effect in subsequent lamination. The lamination, advantageous in itself, is only possible in a working step with mold shaping. In this case however, the lamination cannot be placed around the lateral contour of the molded article, i.e. bent over, so that the field of application of the molded articles is severely limited.
When fibers are used with an extremely low water content, a position from which the prior art proceeds, difficulties arise in heating during production of the semifinished panels, as heat-transmission by means of evaporating water is eliminated. The procedure in which the still-loose fiber mat is permeated during manufacture by a gaseous heating medium is, on the one hand, expensive, and on the other hand, requires flow velocities which lead to fine particles being blown out of the mat. Renewed heating to softening temperature before mold shaping of the semifinished panels in a conventional manner with the mold presses used previously, i.e. preferably with radiant heat, takes too long, and renders the duromeric binders ineffective.
In all, the advantages of a procedure according to DE-OS 28 45 117, compared to the prior art represented by the HMPP technology, are small for the same investment, so that to date economic utilization has not been possible.